Fractional distillation system



Dec. 28, 1965 c. s. KELLEY 3,2

FRACTIONAL DISTILLATION SYSTEM Filed March 4, 1963 2 Sheets-Sheet 2 TV TINVENTOR. C. S. K ELLEY A 7' TORNE Y5 United States Patent 3,225,551FRACTIONAL DIFaTILLATIGN SYSTEM Carl S. Kelley, Bartiesviiie, Okla,assignor to Philiips Petroleum Company, a corporation of Delaware FiledMar. 4, 1963, Ser. No. 262,736 6 Claims. (Cl. 62-21) This inventionrelates to fractional distillation. In one aspect the invention relatesto the control of a fractional distillation operation wherein acomponent to be distilled as overhead product is present in the feedstock in such a low concentration as to provide too small a stream fromthe reflux accumulator to serve for control of fractionator pressure. Inanother aspect the invention relates to a novel heat exchanger-vent gasabsorber.

Prior art fractional distillation has usually treated charge stockscontaining a sufiicient proportion of the lowest boiling component orcomposition so that ample gas flow was obtained for use in regulation offractionator pressure. In one instance, however, when accumulatoroff-gas flow was too small, an extraneous gas was added to the overheadgas stream to increase the off-gas flow. In some instances accumulatorotf-gases have specific utilities and it is undesirable to add anextraneous gas thereto, or to dilute one component of a mixture withadded amounts of another component,

According to this invention, I compress the overhead vaporous streamfrom a fractionator and regulate the reflux accumlator off-gaswithdrawal in response to the pressure dilferential across thecompressor. The cornpressor is operated for a minimum of compression toproduce just sufficient condensate with use of the avail able cooling toreflux the fractionator. In one specific embodiment the availablecooling is the sum of the cooling available from the fractionatorsreboiler and a small amount from a small stream of refrigerant.

The rate of compression is regulated in response to the overheadfractionator pressure. Thus, upon an increase in overhead pressure, thecompressor is speeded up, thereby to return the pressure to its normaloperating pressure. This increase in the rate of compression isaccompanied by an increase in pressure differential across thecompressor. This increase in pressure dilferential then resets the setpoint of a fiow controller on the accumulator off-gas line. The converseof the above is also effected. That is, if the overhead pressuredecreases, the compressor speed is decreased which results in returningthe overhead pressure to normal at a lower compressor speed; thisresults in a decrease in differential pressure across the compressor,and ultimately a decrease in flow of light gases from the system.

Accordingly, it is an object of the invention to provide novel methodand means for controlling a fractional distillation system. Anotherobject of the invention is to provide method and means for regulatingthe overhead pressure in a fractionation column. A still further objectof the invention is to provide method and means for regulating theoverhead pressure in a fractionation system wherein the overhead productis a very small percentage of the feed stream. Yet another object of theinvention is to provide a novel self-refluxing gas absorber.

Other aspects, objects, and advantages of the invention will be apparentfrom a study of the disclosure, the drawing and the appended claims tothe drawing.

In the drawings, FIGURE 1 is a schematic representation of a fractionaldistillation system embodying the control features of the invention;

FIGURE 2 is an elevation view, partly in cross section, of aself-refluxing gas absorber in accordance with the invention; and

FIGURE 3 is a view of the apparatus of FIGURE 2 taken along line 33.

Referring now to the drawing and to FIGURE 1 in particular there isshown a system for the upgrading of the content of a component of a feedstock boiling intermediate the boiling points of the lowest boiling andhighest boiling components of the feed stock wherein the lowest boilingcomponent is present in the feed stock in too low a concentration, forexample on the order of three percent by weight or less of the totalfeed stock, to serve as a means for controlling the pressure in thefractional distillation column. While the invention is applicable to thefractional distillation of any mixture wherein the overhead product ispresent in the feed stock in too low a concentration to serve as a meansfor controlling the pressure in the fractional distillation column, theinvention will be described in terms of a separation of a feed stockcomprising ethane, propylene, and propane as the lowest boilingcomponent, intermediate boiling component, and highest boilingcomponent, respectively. A feed stream of ethane, propylene, and propaneis passed through conduit 11 into a fractional distillation column 13and fractionally distilled therein. An overhead vapor stream containinga portion of the ethane is withdrawn from fractional distillation column13 by way of conduit 15, while a product stream enriched in propylenecan be withdrawn by way of side draw conduit 16. The rate of withdrawalof product by way of conduit 16 can be controlled by any suitable means.For example, the rate of flow through conduit 16 can be controlled by avalve 17 which is manipulated by a flow recorder controller 18responsive to a comparison of a set point value and the differentialpressure across an orifice 19 located in conduit 16. The rate ofwithdrawal of product through conduit 16 can be controlled by valve 17responsive to the output of an analyzer controller 22. If desired theset point value of flow recorder controller 18 can be manipulated byanalyzer recorder 22 responsive to the composition of the product inconduit 16.

A liquid stream is withdrawn from the bottom of fractional distillationcolumn 13 by way of conduit 23 and introduced into reboiler 24 whereinthe withdrawn liquid is subjected to indirect heat exchange to eifectthe vaporization of the liquid. The vapors thus produced are withdrawnfrom reboiler 24 and passed by way of conduit 25 into the lower portionof column 13, preferably below the lowest tray. A vapor streamcontaining substantially all of the propane and only a small portion ofthe propylene is Withdrawn from the bottom section of column 13 by wayof conduit 27 and is passed to a point of utilization, recovery, orfurther treatment. The rate of flow through conduit 27 can be controlledby valve 28 which can be manipulated by liquid level controller 29responsive to the level of liquid kettle material in reboiler 24 tomaintain such level substantially constant. In one particular embodimentthe vapor stream withdrawn by way of conduit 27 is utilized as a fuelgas.

The overhead vapor stream in conduit 15 is introduced into a surge tank30 from which a vapor stream is withdrawn by way of conduit 31 andsubjected to compression in a compressor 33, which is driven by asuitable means such as motor 35. The speed of motor 35, and thus therate of compression, is controlled by a speed control means 37responsive to a comparison of the pressure in conduit 31, as indicatedby a pressure sensing means 39, and a value applied to set point input41. The resulting hot vaporous compressed stream is passed by way ofconduit 43 into reboiler 24 in indirect heat eXchange with the kettlecontents contained therein. The cooled fluid stream is withdrawn by wayof conduit 44 and passed by way of conduit 45 through heat exchanger 46into accumulator 47. If desired, a portion of the cooled fluid streamcan be withdrawn from conduit 44 and bypassed around heat exchanger 46by way of conduit 48. A suitable coolant is passed by way of conduit 49and valve 51 into heat exchanger 46 with the warmed coolant beingwithdrawn by way of conduit 52. Valve 51 can be controlled by pressurerecorder controller 53 responsive to the pressure in accumulator 47 tomaintain such pressure substantially constant by varying the rate offlow of coolant through heat exchanger 46. If desired, the ratio of therates of flow in conduits 45 and 48 can be regulated responsive to thepressure in accumulator 47 to maintain the pressure substantiallyconstant instead of or in addition to the regulation of the rate of flowof coolant through conduit 49. The hot vaporous stream from conduit 43is partially condensed by its passage through reboiler 24 and heatexchanger 46 to produce a condensate and an uncondensed gas enriched inethane. The condensate is withdrawn from accumulator 47 and is passed byway of pump 55 and conduit 56 into an upper portion of column 13 asreflux therefor. The rate of flow of reflux through conduit 56 iscontrolled by valve 57 which is manipulated by flow rate recordercontroller 58 responsive to a comparison of the differential pressureacross an orifice 59 in conduit 56 and the set point value on controller58. The set point value to controller 58 is in turn manipulated byliquid level controller 61, which is mounted on accumulator 47, tomaintain the liquid level in accumulator 47 substantially constant.

A self-refluxing gas absorber 63 is connected to the vapor section ofacumulator 47 and comprises a contactor section 64 and a refrigerationsection 65. A vapor stream is withdrawn from accumulator 47 and passedinto contactor section 64 wherein the ascending vapor is contacted withdescending liquid resulting from a condensation of a portion of theascending vapor in refrigeration section 65. A suitable refrigerant ispassed by way of conduit 66 into indirect heat exchange relationshipwith refrigeration section 65 and is withdrawn therefrom by way ofconduit 67. The remaining unc-ondensed gas is withdrawn from absorber 63and passed by way of conduit 68 and valve 69 to vent, recovery, orfurther treatment. Valve 69 is manipulated by a flow recorder controller71 responsive to a comparison of the differential pressure across anorifice 72 in conduit 68 and a set point value. In accordance with theinvention the set point value to How recorder controller 71 ismanipulated by a differential pressure recorder controller 73 responsiveto the difference in pressure of the overhead vapor stream in conduit 31and the hot vaporous compressed stream in conduit 43. Thus for anincrase in the overhead pressure in column 13 speed control means 37causes motor 35 to speed up and increase the rate of compression incompressor 33 in order to return the pressure in conduit 31 to itsnormal operating value. This increase in the rate of compression isaccompanied by an increase in the diflerential pressure acrosscompressor 33, that is, an increase in the difference in pressurebetween the overhead vapor streamin conduit 31 and the hot vaporouscompressed stream in conduit 43. This increase in pressure differentialis utilized to reset flow recorder controller 71 to permit an increasein the rate of withdrawal of off-gas by way of conduit 68. Similarlyupon a decrease in the overhead pressure in column 13 the speed of themotor 35 is reduced by control means 37, causing a reduction in the rateof compression in compressor 33. This reduction in the rate ofcompression is accompanied by a decrease in differential pressure acrosscompressor 33, and this decrease in pressure differential is utilized toreset flow recorder controller 71 to reduce the rate of withdrawal ofolT-gas by way of conduit 68. Accordingly it is readily seen that thecontrol system of the invention provides for maintaining the overheadpressure of the fractional distillation column substantially constant atthe desired value while regulating the rate of flow of ofl-gas withoutincurring any excessive loss of propylene through conduit 68. Thiscontrol system reduces the consumption of power by the compressor to aminimum, decreases possibility of ofi-specification product, andrelieves the operator from having to continuously watch and resetcontrols.

A first signal representative of the rate of flow of fluid throughconduit 27 is transmitted by way of line 75 to a first input ofdifferential flow rate recorder controller 76. A second signalrepresentative of the rate of flow of fluid through conduit 25 istransmitted by way of line 77 to a second input of controller 76.Controller 76 produces an output signal responsive to the difierencebetween the first and second signals. The output signal is transmittedby way of line 78 to the reset input of pressure recorder controller 79.A valve 81 in conduit 44 is manipulated by pressure recorder controller79 responsive to a comparison of the actual pressure in conduit 44 andthe output signal of controller 76. Valve 81 thus controls the amount ofheat which is transferred from the hot vaporous compressed stream ofconduit 43 into the kettle material in reboiler 24, and thereby the heatinput to column 13. For example, if too much liquid is produced in thebottom of column 13, the liquid level of kettle material in reboiler 24will tend to rise. Liquid level controller 29 will actuate valve 28 toincrease the rate of withdrawal of vapor through conduit 27. Thisresults in a decrease in the difference between the flow rates inconduits 25 and 27, and differential flow rate controller 76 accordinglyresets the set point value on pressure controller 79 to actuate valve 81to raise the pressure upstream of valve 81. This increase in pressure ofthe compressed vapors passing from conduit 43 into reboiler 24 raisesthe condensation pressure and hence the condensation temperature of thecompressed vapors. This permits additional heat to be transferred fromthe compressed vapors to the kettle material in reboiler 24 due to thehigher differential temperature. The increase in heat transferredresults in an increase in flow rate through conduit 25 until the systemreturns to desired operating conditions. A reverse action, that is, adrop in the liquid level in reboiler 24, will cause the reverse effect.Consequently the control system regulates the heat input to the columnand thereby stabilizes operation of the column.

Referring now to FIGURE 2 self-refluxing gas absorber 63 comprisescontacting section 64, refrigeration section 65 and cap 90. Contactingsection 64 comprises a vertically positioned cylindrical vessel 91having its lower end in vapor communication with accumulator 47, and oneor more liquid-vapor contacting means 92, such as bubble cap tray, sievetray or plate tray, positioned within vessel 91. Refrigeration section65 comprises a vertically positioned cylindrical vessel 93, a perforateplate 94 positioned across the entire diameter of vessel 93 and mountedbetween vessels 91 and 93, a perforate plate positioned across theentire diameter of vessel 93 and mounted between vessel 93 and cap 90,and a plurality of separate tubes extending in fluid-tight relationshipfrom each perforation in lower plate 94 into corresponding perforationsin upper plate 95. The upper ends of the tubes 96 are substantiallyflush with the upper surface of plate 95 while the upper ends of tubes97 extend above plate 95 a suitable distance, for example on the orderof one inch. Tubes 96 and 97 are each spaced from one another to providefree space between them into which the coolant is passed by way ofconduit 66 with the warmed coolant being withdrawn by way of conduit 67.While a plurality of tubes 96 have been illustrated in the drawing, itis within the contemplation of the invention to utilize one or moretubes 96. The vapors ascending through section 91 are contacted with thedownflowing liquid which is obtained by condensation of a portion of thevapors passing through tubes 96 and 97. Substantially all of theentrained liquid in the vapor streams leaving tubes 96 and 97 isseparated out in cap 90 and falls on plate 95. The liquid collecting onplate 95 drains through tubes 96, thereby preventing any flooding oftubes 97.

The following example is presented in further illustration of theinvention and should not be construed unduly in limitation thereof. Inthe operation of a particular fractional distillation system inaccordance with the invention for the separation of a feedstockcomprising ethane, propane, and propylene, under the conditions setforth in Table I, the process streams have the compositions set forth inTable II.

Table 1 Pressure, Temperature,

p.s.i.g. F.

Feed Stock 110 45 Column 13:

Kettle 91 55 Overhead- 83 38 Surge Tank 30- 8O 38 Compressor 33:

Suction 80 38 Discharge 135 95 Conduit 44 132 68 Accumulator 47 130 64Conduit 68 129 15 Conduit l6..- 85 45 91 55 91. 5 57 91 55 Table II 1116 27 68 Component/Stream Lbs/hr. Lbs/hr. Lbs/hr Lbs/hr Ethane 150 5 145Propylene 17, 000 16, 155 700 145 Propane 9, 850 S0 9, 770

Total 27, 000 16, 240 10, 470 290 The effects of a sli ht increase in Ccontent of feed stream is illustrated in Table III.

The slight increase in C content illustrated in Table III requires thatalmost twice as much off-gas be vented. The control system of theinvention maintains the loss of propylene through conduit 68 at aminimum while maintaining the power consumption of compressor 33 at aminimum.

Reasonable variations and modifications are possible within the scope ofthe foregoing disclosure, the drawings, and the appended claims to theinvention.

I claim:

1. A process for the regulation of pressure in a fractional distillationzone operating for the purification of a feed component having a boilingpoint intermediate the boiling points of the lowest boiling and highestboiling components of the feed to be distilled and wherein the lowestboiling component is present in said feed in too low a concentration toserve as a means for regulation of pressure in said fractionaldistillation zone, comprising fractionally distilling said feed in saidfractional distillation zone thereby producing an overhead vapor streamcontaining the major portion of said lowest boiling component,compressing this overhead vapor stream thereby producing a hot vaporouscompressed stream, partially condensing this hot vaporous compressedstream by indirect heat exchange with kettle contents of said zone andby further indirect heat exchange with a coolant thereby leavinguncondensed gas enriched in said lowest boiling component, separatingthe resulting condensate from said uncondensed gas, returning thecondensate to the distilling step as reflux, sensing the differentialpressure between said overhead vapor stream and said hot vaporouscompressed stream, and regulating the rate of withdrawal of saiduncondensed gas in response to the sensed differential pressure.

2. The process of claim 1 wherein the lowest, intermediate and highestboiling components are ethane, propylene, and propane, respectively.

3. A process for the regulation of pressure in a fractional distillationzone operating for the upgrading of the content of a component of a feedstock boiling intermediate the boiling points of the lowest boiling andhighest boiling components of the feed to be distilled, said feed beingrich in the intermediate boiling component, and the lowest boilingcomponent being present in the feed in too low a concentration to serveas a means for control of pressure in said fractional distillation zone,comprising fractionally distilling said feed in said fractionaldistillation zone thereby producing an overhead vapor stream containingthe major portion of said lowest boiling component, compressing thisoverhead vapor stream thereby producing a hot vaporous compressedstream, partially condensing this hot stream by indirect heat exchangewith kettle contents of said fractional distillation zone and by furtherindirect heat exchange with a coolant thereby leaving an uncondensed gasenriched in said lowest boiling component, separating the result ingcondensate from said uncondensed gas, returning said condensate to thefractional distillation zone as reflux, sensing the pressuredifferential between said overhead vapor stream and said hot vaporouscompressed stream, withdrawing the uncondensed gas from the system inresponse to the sensed pressure differential, sensing the pressure ofsaid overhead vaporous stream, regulating the rate of compressing inresponse to the sensed pressure, and withdrawing a sidestream from saidfractional distillation zone, said sidestream being richer in saidintermediate boiling component than the feed stock.

4. The process of claim 3 wherein the lowest, intermediate, and highestboiling components are ethane, proplene, and propane, respectively.

5. A process for the regulation of pressure in a fractional distillationzone operating for the upgrading of the content of a component of a feedstock boiling intermediate the boiling points of the lowest and highestboiling components of the feed to be distilled, said feed containingfrom an infinitesimal amount up to about 3 percent by weight of thelowest boiling components, said amount of lowest boiling componentsbeing too small to provide sufficient accumulator gas flow for pressurecontrol of the fractional distillation zone, comprising fractionallydistilling said feed stock in said fractional distillation zone therebyproducing an overhead vaporous stream containing the major portion ofsaid lowest boiling components, compressing this overhead vapor streamthereby producing a hot vaporous compressed stream, partially condensingthis hot vaporous compressed stream by indirect heat exchange withkettle contents of said zone and by further indirect heat exchange witha coolant thereby leaving an uncondensed gas enriched in said lowestboiling components, separating the resulting condensate from saiduncondensed gas, returning said condensate to the fractionaldistillation zone as reflux therefor, sensing the pressure differentialbetween said overhead .vaporous stream and said hot vaporous compressedstream, withdrawing the uncondensed gas in response to the sensedpressure differential, sensing the pressure of said overhead vaporousstream, regulating the rate of compressing in response to the sensedpressure, and Withdrawing a sidestream from said distillation zone, saidsidestream being richer in said intermediate boiling component than saidfeed stock.

6. The process of claim 5 wherein said lowest boiling constituentscomprise ethane and ethylene, and said intermediate and highest boilingconstituents comprise a major proportion of propylene and a minorproportion of propane.

References Cited by the Examiner UNITED STATES PATENTS Schlitt 6226 XRHaynes 6226 XR Haldernan 261153 Hackmuth 6231 XR Moore 261-153 Sattler6221 Greca 62-31 XR Cabbage 6221 NORMAN YUDKOFF, Primary Examiner.

1. A PROCESS FOR THE REGULATION OF PRESSURE IN A FRACTIONAL DISTILLATIONZONE OPERATING FOR THE PURIFICATION OF A FEED COMPONENT HAVING A BOILINGPOINT INTERMEDIATE THE BOILING POINTS OF THE LOWEST BOILING AND HIGHESTBOILING COMPONENTS OF THE FEED TO BE DISTILLED AND WHEREIN THE LOWESTBOILING COMPONENT IS PRESENT IN SAID FEED IN TOO LOW A CONCENTRATION TOSERVE AS A MEANS FOR REGULATIONS OF PRESSURE IN SAID FRACTIONALDISTILLATION ZONE, COMPRISING FRACTIONALLY DISTILLING SAID FEED IN SAIDFRACTIONAL DISTILLATION ZONE THEREBY PRODUCING AN OVERHEAD VAPOR STREAMCONTAINING THE MAJOR PORTION OF SAID LOWEST BOILING COMPONENT,COMPRESSING THIS OVERHEAD VAPOR STREAM THEREBY PRODUCING A HOT VAPOROUSCOMPRESSED STREAM, PARTIALLY CONDENSING THIS HOT VAPOROUS COMPRESSEDSTREAM BY INDIRECT HEAT EXCHANGE WITH KETTLE CONTENTS OF SAID ZONE ANDBY FURTHER INDIRECT HEAT EXCHANGE WITH A COOLANT THEREBY LEAVINGUNCONDENSED GAS ENRICHED IN SAID LOWEST BOILING COMPONENT, SEPARATINGTHE RESULTING CONDENSATE FROM SAID UNCONDENSED GAS, RETURNING THECONDENSATE TO THE DISTILLING STEP AS REFLUX, SENSING THE DIFFERENTIALPRESSURE BETWEEN SAID OVERHEAD VAPOR STREAM AND SAID HOT VAPOROUSCOMPRESSED STREAM, AND REGULATING THE RATE OF WITHDRAWAL OF SAIDUNCONDENSED GAS IN RESPONSE TO THE SENSED DIFFERENTIAL PRESSURE.